Polyester resins may be produced by the esterification of aromatic acids with various glycols. For example, the esterification of terephthalic acid (TA) with ethylene glycol (EG) produces polyethylene terephthalate (PET), a widely used polyester resin in, for example, the food packaging and textile fiber industries.
PET is a linear polyester which is generally manufactured in two stages by (1) esterification of terephthalic acid (TPA) with an excess of ethylene glycol (EG) or by the ester exchange reaction of dimethyl terephthalate (DMT) and an excess of EG to form dihydroxyethyl terephthalate (DHET), and (2) the polycondensation of DHET in the presence of a metal oxide catalyst. The metal oxide catalyst is typically an oxide of antimony or germanium.
The first stage ester reaction requires an excess of EG. The excess EG is removed during the course of the polycondensation reaction along with other products such as low molecular weight terephthalate oligomers, diethylene glycol (DEG), metal oxide catalysts and trace amounts of other compounds. The EG containing impurities is hereinafter referred to as spent glycol (SG). The presence of impurities in the spent glycol prevents recycling of the spent glycol into the first stage esterification since product quality would be detrimentally affected thereby. In particular, when product having little or no color and free of insoluble particulate matter is required, spent glycol is unsuitable for recycling.
The prior art on recycling spent glycol relies primarily on flash distillation of the spent glycol as typified by U.S. Pat. Nos. 3,408,268, 3,367,847 and 2,788,373. There are numerous variations to the basic distillation process. For example, U.S. Pat. No. 3,878,055 teaches flash distillation of spent glycol in the presence of an alkali metal hydroxide, while U.S. Pat. No. 3,491,161 teaches the addition of ammonium hydroxide prior to distillation. Some attempts have been made to remove antimony by precipitation prior to distillation of the spent glycol. Typical of these processes are U.S. Pat. Nos. 4,118,582 and 4,013,519.
In practice, spent glycol is purified by distillation in which a pure EG overhead product is recovered. The refined EG is typically recycled to the esterification process. The still bottoms resulting from the distillation of the spent glycol is a mixture of antimony or germanium oxide catalyst, titanium dioxide, optical enhancers, terephthalate oligomers, EG and DEG and various trace impurities such as trace cations, trace anions and color forming impurities.
Significantly, large quantities of still bottoms are generated in the United States each year which presents serious environmental as well as economic problems for PET producers. Furthermore, disposal of the still bottoms as a waste product represents the loss of substantial quantities of EG, DEG, catalysts, and terephthalate oligomers, all of which have commercial value.
There is therefore, a need for a PET manufacturing process which removes contaminants from spent glycol and which allows for the recovery and recycle of the still bottoms. Especially desirable is an improved process in which all materials are recycled to either the PET manufacturing process or to other chemical manufacturing processes.
Other types of resins are manufactured using other aromatic acids and glycols. Other examples of aromatic acids include terephthalic acid, isophthalic acid, cyclohexane dicarboxylic acid, and 2,6-naphthalene dicarboxylic acid. Examples of glycols include ethylene glycol, diethylene glycol, 1,3-propanediol, and 1,4-butanediol. Other resins may be produced from the transesterification of methyl esters of aromatic acids with certain glycols. Examples of resins made with this process include dimethyl terephthalate with EG, and 2,6-dimethyl naphthalate with EG which produces polyethylene naphthalate (PEN) resins.
In general in the typical process for the production of polyester resins, the ester is initially produced from the reaction of an aromatic acid and a glycol. (The ester is the monomer from which the polymer is produced.) Glycol is produced during the polycondensation reaction of the ester. During the polycondensation step the excess glycol is removed from the reactor as a vapor along with other materials, including monomer and higher molecular weight oligomer, metal catalyst, delusterants such as titanium dioxide (TiO.sub.2), and additives for optical enhancement.
As noted above, the SG is typically recovered and purified by conventional distillation methods in which the glycol is recovered as an overhead distillate product. The higher boiling impurities are removed as still bottoms. The still bottoms are typically disposed of by incineration. The incineration process is very expensive in terms of lost monomer and glycols and the energy required to distill the glycol. The incineration process also produces considerable quantities of fly ash containing hazardous metals which results in an environmental disposal problem. There is therefore a need for an economical method to remove particulate matter from SG and to recycle the recovered glycol to the esterification process which avoids the costly, wasteful and environmentally detrimental distillation and incineration steps.
These and other problems of the prior art are solved by the present invention as described more fully below.